The present invention relates generally to systems for the transmission and reception of high speed data signals between a central station and a plurality of remote units using a discrete multi-tone (DMT) multi-carrier approach. More particularly, the use of a dedicated overhead bus for synchronizing frames transmitted from the various remote units is described.
At the time of this writing, the Alliance For Telecommunications Information Solutions (ATIS), which is a group accredited by the ANSI (American National Standard Institute) Standard Group, is nearing finalization of a standard for the transmission of digital data over Asymmetric Digital Subscriber Lines (ADSL). The standard is intended primarily for transmitting video data over ordinary telephone lines, although it may be used in a variety of other applications as well. The standard is based on a discrete multi-tone transmission system. The pending North American Standard is referred to as the T1E1.4 ATIS Standard, and is presently set forth in Standard Contribution No. 94-007, rev. 2, dated April of 1994, which is incorporated herein in its entirety. Transmission rates are intended to facilitate the transmission of information at rates of at least 6 million bits per second (i.e., 6+Mbps) over ordinary phones lines, including twisted-pair phone lines. The standardized discrete multi-tone (DMT) system uses 256 "tones" that are each 4.3125 kHz wide in the forward (downstream) direction. That is, in the context of a phone system, from the central office (typically owned by the telephone company) to a remote location that may be an end-user (i.e., a residence or business user).
The Asymmetric Digital Subscriber Lines standard also contemplates the use of a duplexed reverse signal at a data rate of 16-800 Kbps. That is, transmission in an upstream direction, as for example, from the remote location to the central office. Thus, the term Asymmetric Digital Subscriber Line comes from the fact that the data transmission rate is substantially higher in the forward direction than in the reverse direction. This is particularly useful in systems that are intended to transmit video programming or video conferencing information to a remote location over the telephone lines. By way of example, one potential use for the systems allows residential customers to obtain videos information such as movies over the telephone lines rather than having to rent video cassettes. Another potential use is in video conferencing.
The discrete multi-tone (DMT) transmission scheme has the potential for use in applications well beyond data transmissions over telephone lines. Indeed, DMT can be used in a variety of other digital subscriber access systems as well. For example, it may be used in cable based subscriber systems (which typically use coaxial cable) and wireless subscriber systems such as digital cellular TV. In cable systems, a single central unit (central modem) is typically used to distribute digital signals to more than one customer which means more than one remote unit (remote modem). While all of the remote modems can reliably receive the same digital signals, the upstream transmissions must be coordinated to prevent confusion at the central modem as to the source of the upstream signals. Presently, in cable systems (which do not use discrete multi-tone transmission schemes), each remote unit is given a dedicated frequency band over which it is to communicate with the central station. However, such an approach is inherently an inefficient use of transmission bandwidth and typically requires the use of analog filters to separate transmissions from the various remote units. Stationary digital cellular transmission systems face similar obstacles.
ADSL applications have the potential for a similar problem, although it is typically more limited in nature. Specifically, a single line may service a plurality of drop points at a particular billing address (which may typically be a home or an office). That is, there may be several telephone "jacks" through which the user may wish to receive signals. To facilitate service to multiple locations (jacks) over a single line, the use of a master modem has been proposed to facilitate synchronization. However, this is perceived as being a relatively expensive and undesirable solution. Accordingly, it would be desirable to provide a mechanism in discrete multi-tone data transmission systems which facilitates the synchronization of signals from a plurality of remotes so that a central unit can coordinate and reliably interpret signals sent from the remotes.